U.S. Pat. No. 3,485,099 discloses a fluid flow responsive apparatus comprising a thermocouple having both its junctions in the fluid, and means for periodically heating one of the junctions. An output circuit responds to the thermocouple EMF produced between the heating periods. The thermocouple EMF depends on the rate of flow of the loss of heat at the heated junction and depends thus on the rate of flow of the fluid. Similar apparatuses are disclosed in DE-A No. 1,928,228, DE-A No. 1,648,006, U.S. Pat. No. 1,996,943, and U.S. Pat. No. 4,240,441. Typically the flowmeters comprise a probe including one or two thermocouples and a heating coil and reference means connected through a cable to associated electronic equipment including a current source, a control means, switching means, and measuring means.
Especially the medical industry presents a demand for very small probes and connecting cables to the flowmeters. All the known systems require several electrically conducting wires between the probe of the flowmeter and the associated electronic. Typically eight conductors are used in a connecting cable between a probe and the electronic equipment, as the heating coil, a reference means, and an active sensor all need connections including connections to the ground. In addition a demand has arisen for a diminution and preferably a complete omission of a supply of heat.